When drilling an oil or gas well, or when refurbishing an existing well, normal operations may result in various types of metal debris in a well. Downhole milling produces cuttings which often are not completely removed by circulation. Other metallic objects may drop into and collect near the bottom of the well, or on intermediate plugs placed within the well.
Methods have been used to circulate fluids up the annulus at a rapid rate and thereby carry debris upward, with expectations that the debris will then settle into the basket for retrieval when circulation is reduced. Some basket tools utilize a venturi action to drawn debris into the tool.
Other tools utilize magnets mounted within a housing for being lowered into the well. Some tools practically may be limited to retrieving cutting since magnetization is only at the bottom of the tool. Other tools utilize a plurality of magnets aligned in cavities near the outer surface of the tool. Each magnet may be recessed in the tool body. Exposed magnets are subject to physical damage during the process of cleaning debris from the well. Conventional metal debris retrieving tools are relatively expensive, and it is difficult or impossible to effectively clean and change out the magnets of most tools in the field.
U.S. Pat. No. 6,655,462 discloses magnets within an interior of a rotatable protective sleeve. U.S. Pat. Nos. 6,216,787, 6,308,781, and 6,491,117 disclose magnetic tools and magnet protectors for deflecting a striking force on the magnets. U.S. Pat. No. 6,439,303 discloses the retainer caps for mounting each of a plurality of magnets within a recess in the tool body. Publication 2001/4013413 discloses in one embodiment a magnet which covers substantially the entire surface of the tool body. Other patents of interest include U.S. Pat. Nos. 2,089,724, 2,709,104, 2,729,494 and 2,918,323, 3,905,631, 4,226,285, 5,178,757, 5,453,188, 5,944,100, 6,269,877, and 6,629,562.
The disadvantages of the prior art are overcome by the present invention, and an improved magnetic retrieval tool and method are hereinafter disclosed for retrieving debris from a well.